Why Flex with Rails?

First it's not an all or nothing, you can use Flex in your Rails application only where it makes sense. Unless you have a reason to look for an approach other than HTML/JavaScript and CSS, don't look into Flex. But if you start spending lots of time in JavaScript,
if you have to transfer tons of data, if you have to do some advanced data visualization, if you have to
implement some fancy product configurator, if your are creating an admin tool, if your application is
geared towards data entry, if you need tighter integration with the desktop...
it may be time for you to check out Flex and AIR on Rails.